This study is designed to further understanding of certain aspects of the cellular and molecular control of biological calcification. We have previously shown that undifferentiated chick limb mesodermal cells can differentiate into chondrogenic cells in culture. Preliminary experiments show that such cell cultures eventually exhibit properties characteristic of osteogenic cells. The two most convincing measurements to date show that Ca ions binding and parathyroid hormone (PTH) binding are absent earlier in the culture period during the time of major chondrogenic commitment and appear later at a time correlated with the decline of chondrogenic activity. Studies proposed here attempt to solidify and extend this preliminary evidence in an attempt to confirm that osteogenic cells arise in such cultures. We plan to look at the temporal aspects of: 1) collagen synthesis, assaying for bone-versus cartilage-specific molecules; 2) Ca ions and related phosphate binding properties in order to analyze for calcium apetite formation; 3) the acquisition of PTH binding capacity to determine the role of PTH in regulation of bone formation; 4) parathyroid hormone-sensitive biochemical responses characteristic of bone cells (such as adenylate cyclase activity effects on carbohydrate metabolism and nucleic acid metabolism); and 5) light and electron microscopic analysis using histochemical and autoradiographic techniques in order to correlate the biochemical analysis with the morphological aspects of bone development. We plan to manipulate the cell culture system using various teratogens, metabolic inhibitors and different environmental conditions in order to understand how aberrant development affects the interrelated pattern of events leading to osteogenic cell formation and biological calcification. Some of the chemicals and environmental conditions to be used not only cause lesions in chick limb cells but also cause cranial-facial as well as other skeletal anomalies. We believe this cell culture system can be used as a model for understanding the control features of other biological systems involved in osteogenic activity.